This invention relates to partially reflective coatings useful for controlling thermal radiation, that is infrared reflective coatings that that have selectively controlled optical properties in visual wavelengths permitting a wide variation in perceived color and brightness having optimal infrared optical properties.
Various methods have been used to achieve thermal radiation control of objects by selectively controlling the object's reflectivity to infrared radiation. Methods generally involve either applying a coating to an object or forming its outer surface of a material having high infrared reflectivity. Thermal radiation controlled objects and surfaces have a number of uses, among which are solar collector absorber panels, space vehicle surfaces, and camouflaging military vehicles from detection by infrared scanning. In many cases, it is desirable to selectively reflect specific wavelengths of the infrared radiation while attenuating others by absorption. For example, for solar energy collection it is desirable that the surface coating absorb radiation corresponding to the sun's solar emission spectrum, that is principally from 300 to 2,500 nanometers, while having a higher reflectivity at longer (generally above about 4 microns) “thermal” wavelengths. This allows the object to absorb and retain the sun's heat because the increased reflectivity at the thermal wavelengths decreases emittance of these wavelengths. The temperature of the object generally determines the wavelength of the thermal emissions. For an object in thermal equilibrium with its surroundings having a given temperature, the term emmitance is defined at the ratio of the energy emitted by such object divided by the energy that would be emitted by a perfect black body at the same temperature. For an object at room temperature (27 degrees Celsius, 300 degrees Kelvin), emittance can be written as:       ɛ          300      ⁢      K        =                    ∫        4        40            ⁢                        (                      1            -                          R              ⁡                              (                λ                )                                      -                          T              ⁡                              (                λ                )                                              )                ⁢                  BB          ⁡                      (                          λ              ,                              300                ⁢                K                                      )                          ⁢                                  ⁢                  ⅆ          λ                                    ∫        4        40            ⁢                        BB          ⁢                                          (                      λ            ,                          300              ⁢              K                                )                ⁢                  ⅆ          λ                    where R(λ) and T(λ) are the reflectance and transmittance of the object at each wavelength, respectively, BB(λ, 300K) is shorthand for the so-called black-body function predicting the amount of energy emitted by a 300°K perfect black body at each wavelength, and the limits of integration are 4 micrometers to 40 micrometers. For an opaque object, the transmittance is zero and hence the higher the IR reflectance of the object, the lower the IR emittance. Conversely, for opaque objects, the lower the IR reflectance of the object, the higher the IR emittance.
Differential absorption and reflectivity of solar and thermal wavelengths, can be achieved by first metallizing a surface with highly polished or reflective metal foils or coatings, as many metals are highly reflective in the solar and far-infrared (thermal) regions of the spectrum. Selective absorption in the solar wavelengths can be achieved by over coating the reflective metallic surface with materials that selectively absorb solar wavelengths. The most common methods for forming such surfaces are by electrochemical deposition techniques followed by chemical oxidation of the deposit and by “paint” technology using organic based coatings.
In the former case, a suitable substrate, such as aluminum, is electroplated with copper. The copper surface is then chemically oxidized to form a surface layer of cupric oxide. One objection to this method is the high cost of the combined electrochemical/chemical oxidation process to obtain the desired surface. Another disadvantage is that one cannot select the visible color or appearance of the composite structure, which is the color of cupric oxide.
Paint technology has been used to give objects selective radiation properties. Some paints use organic solvents and organic binders with an additive. A particular example is a lead sulfide/silicone resin binder in xylene. However, organic-based paints typically release volatile organic compounds, which may be controlled or even prohibited in some areas because of environmental concerns. Further, such organic paints may not provide sufficient radiation control properties for some applications.
Water-based paints have also been investigated. Black silicate paint has been developed that uses a suitable pigment bound in an alkali metal silicate, such as sodium silicate. Such a formulation is sprayable and achieves an effect similar to that of the electrochemical process or by organic binder paint technology. Unfortunately, the pigments can react chemically with the silicate binders in some instances to form pigment silicate salts or complexes. These compounds alter the properties of absorptance and emittance of the coating to such an extent that the performance of the coating may degrade to an unacceptable level.
Another approach is to use two water-based coatings in which a layer of a semiconductor pigment is first deposited upon a thermally reflective substrate and then this pigment layer is overcoated with an alkali metal silicate binder. The silicate layer is heat cured at above ambient temperatures to form a protective coating over the pigment.
The technology of thermal radiation control surfaces is based on the need to obtain a surface that absorbs radiation in the range of 300 to 2,500 nanometers while at the same time suppressing emission of thermal energy. This basic principle accounts for the operation of solar collector absorber panels, infrared transparent coatings used on military equipment and the like. The general approach is to start with a substrate material that has high reflectance (low absorbance) over the entire spectral range including the incident radiation and potential emission (300 to 40,000 nanometers, for example). Examples of such useful substrates include metals such as aluminum, copper, steel and the like, and non-metallic substrates, such as plastics and glass, which can be metallized to provide a highly reflective surface.
In order to obtain the desired properties of opaqueness to ultraviolet and visible light and transparency to infrared radiation, it is desirable to form a coating on the highly reflective substrate that absorbs in the visible and ultraviolet region while transmitting infrared. The combination of the coating on the substrate is preferably highly absorbing in the visible range and highly reflecting (low emitting) in the thermal range. This makes semiconductor pigments highly desirable, as these compounds are highly transparent in the infrared, but absorb in the visible region. Not all semiconductor pigments are useful, as those having a high refractive index and thus a high surface reflection coefficient give rise to unacceptable reflection losses. Thus, only those semiconductor pigments having low enough refractive indices to keep surface reflectivity at a minimum are acceptable. Among such useful semiconductor pigments are copper oxide, iron oxides, both naturally-occurring and synthetically made, chromium oxides, nickel oxide, complexes of nickel-zinc-sulfide, lead sulfide and so forth. Since thermal and photochemical stability is required of the semiconductor, organic dyes would not be very useful and the preferred semiconductors are, therefore, the inorganic pigments already enumerated.
Such thermal control surfaces have undesirable visual appearances for many applications because the broadband reflectors are very bright or metallic in appearance, while selective absorbers have a black appearance in the visible.
One approach uses a paint composition to achieve a diffused visual blue-gray coating of non-metallic texture for use on metal surfaces that provides reduced infrared emittance. Previous camouflage coatings and paints used on hulls of naval vessels often exhibited relatively high solar absorption because of the dark colors and diffused finishes that are characteristic of the coatings. This high solar absorption resulted in high surface temperatures that increase cooling requirements and more importantly increased infrared radiation. In modern warfare, infrared detection techniques have become highly developed and means for counter-detection techniques are accordingly required. Artificial cooling of hot exposed surfaces is effective to reduce infrared emission. However, this method increases electrical power requirements aboard ship as well as adding parasitic weight and volume to equipment aboard the ship.
This low infrared emittance coating is applied as a paint to provide a durable opaque coating suitable for use on exposed surfaces of naval vessels or on hot surfaces of a gas turbine exhaust. Ideally, such coated surfaces exhibit low reflectance in the visual portion of the light wavelengths and high reflectance in the infrared portion. The paint is a mixture of colorant and emitance control pigments such as aluminum, zinc sulfide, antimony trisulfide, and blue pigments; aluminum oxide filler; silicon alkyd resin binder; polarized montmorillite clay; and a diluent. Like traditional military paints, it utilizes some fraction of visually absorptive pigments, that do not have wavelength specific or optimized infrared properties; their reflectance tends to be constant over different wavelength bands, which compromises its infrared performance.
Various flakes or pigments have been made that have optically selective or optically variable properties. Some optically selective pigments have interference structures that enhance or suppress a portion of the visible spectrum to achieve a desired color, and are generally used in colorful paints, inks, plastics, and other carriers. Some optically variable pigments are similarly directed at the visible spectrum and shift color as the location of the observer changes. The interference structures typically include thin film layers of spacer (dielectric) and absorber materials over a reflector. Similar flakes utilize optical coating structures to selectively absorb solar radiation in paint intended for passive solar energy systems; however, the thermal emittance characteristics of the paint appears to be influenced by the infrared absorption spectra of the paint vehicle.
Prior technology for thermal control of visually opaque objects resulted in either a highly reflective metallic appearance, in the case of broadband visual IR reflectors, or a black color, in that materials selectively absorbing in the solar IR region are also absorbing in visual wavelengths. Other attempts at reducing the chroma of coatings often include adding a darkening agent to the carrier or binder. This often detracts from the IR performance of the coating. Attempts to achieve other visual colors resulted in some compromise of the thermal control properties.
Accordingly, it would be desirable to provide flake-like pigments that have low reflection in visible light in a range of colors, with selectable visible color characteristics being independent from their IR characteristics.
Another object of the present invention is to provide thermal control of visually opaque objects, especially those having arbitrary or irregular shape by application of a coating or foil that allows selective absorption of light at visible wavelengths, and reflection of light at IR wavelengths. It is further desirable that thermal control coatings have a range of independently selected color and chroma in the visible wavelengths.
Another objective of the present invention to provide efficient solar energy collection absorbing material with low thermal emittance, and that solar energy collection absorbing material be available in a selection of colors.